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One-directional electron transport between a photocatalyst and redox mediator is crucial for achieving highly active Z-scheme water-splitting photocatalysis. Herein, a photoredox cascade catalyst that artificially mimics the electron transport chain in natural photosynthesis was synthesized from a Pt-TiO2 nanoparticle catalyst, two photosensitizers (RuCP6 and RuP6), and a visiblelight-transparent electron mediator (HCRu). During photocatalytic hydrogen evolution in the presence of a redox-reversible electron donor, [Co(bpy)3]2+ (bpy = 2,2 '-bipyridine), the HCRu-Zr-RuCP6-Zr-RuP6@Pt-TiO2 (PRCC-1) photocatalyst exhibited the highest reported initial (1 h) apparent quantum yield (iAQY = 2.23%) of dye-sensitized TiO2 photocatalysts to date. Furthermore, PRCC-1 successfully produced hydrogen when using hydroquinone monosulfonate (H2QS-) as the hydrogen source.

One-directional electron transfer is crucial for two-step photoexcitation (Z-scheme) water-splitting photocatalysis. We investigated the hydrogen evolution activity of PS-double-layered photocatalysts (X-DSP, X-RuCP6-Zr-RuP6@Pt/KxH4-xNb6O17; X=Zr4+, H+) in the presence of highly charged polyoxometalates-K-6[(SiVW11O40)-W-IV] center dot nH(2)O (V-IV-POM) and K-6[SiW11O39MnII(H2O)] center dot nH(2)O (Mn-II-POM)-as redox-reversible electron donors, to induce effective photocatalyst-donor electrostatic attraction. Surface-phosphonate-comprising H+-DSP completely one-electron oxidized V-IV-POM with 0.39 % apparent quantum yield in the initial hour (iAQY) in both HCl and phosphate buffer aqueous solutions. Conversely, the Zr4+-DSP iAQY decreased to 0.05 % in HCl aq. Considering that the 0.39 % iAQY was retained when replacing V-IV-POM with Mn-II-POM in HCl aq, we supposed that energy transfer deactivation from photoexcited PS* to surface-immobilized V-IV-POM is the plausible origin of the lower iAQY, owing to the stronger visible-light absorptivity of V-IV-POM. This suggests that accumulation of visible-light-transparent electron mediator on the photocatalyst surface is an effective approach for one-directional electron transfer in Z-scheme water-splitting photocatalysis.

Trihexyltetradecylphosphonium (P-6,P-6,P-6,P-14) salts of luminescent anionic Pt(II) complexes, (P-6,P-6,P-6,P-14)[PtX2(ppy)] (X = Cl-, Br-; ppy = 2-phenylpyridinate), are synthesized and photofunctional thin films with crystal/liquid bi-stability are fabricated. In particular, the chloride complex provides a thin film exhibiting thermo- and mechano-triggered luminescence ON/OFF switching at ambient temperature, which is based on the control of the supercooled liquid phase and bright luminescent crystalline phase. The photophysical properties of the complexes are investigated and compared with those of the bromide complex and tetra(n-butyl)ammonium salts of the complexes. Furthermore, detailed photophysical analysis reveals a large contribution of the charge-transfer character to the ligand-centered (3)pi pi* excited state, which results in intense phosphorescence in the crystal and high-contrast luminescence ON/OFF by the phase transition of the chloride complex.

To develop highly active H2 evolving dye-sensitized photocatalysts (DSPs) applicable for Z-scheme water splitting, we synthesized a series of Ru(II)-dye-double-layered DSPs, X '- RuCP6-Zr-RuP6@Pt-TiO2 (X '-DSP) with different surface bound metal cations (X ' = Fe2+, Y3+, Zr4+, Hf4+, and Bi3+). In 0.5 M KI aqueous solution, the photocatalytic H2 evolution activity under blue light irradiation (lambda = 460 +/- 15 nm) increased in the following order: nonmetal-modified DSP, H+-DSP (turn over number for 6 h irradiation = 35.2) < Fe2+-DSP (54.9) asymptotic to Bi3+-DSP (55.2) < Hf4+-DSP (65.5)asymptotic to Zr4+-DSP (68.3)asymptotic to Y3+-DSP (71.5), suggesting that the redox-inactive and highly charged metal cations tend to improve the electron donation from the iodide electron mediator. On the other hand, DSPs having heavy metal cations, Hf4+-DSP (18.4) and Bi3+-DSP (16.6), exhibited better activity under green light irradiation (lambda = 530 +/- 15 nm) than Zr4+-DSP (15.7) and H+-DSP (7.80), implying the contribution of a heavy atom effect of the surface-bound metal cation to partially allow the spin-forbidden metal-to-ligand charge-transfer excitation.

Trihexyltetradecylphosphonium (P-6,P-6,P-6,P-14) salts of luminescent anionic Pt(II) complexes, (P-6,P-6,P-6,P-14)[PtX2(ppy)] (X = Cl-, Br-; ppy = 2-phenylpyridinate), are synthesized and photofunctional thin films with crystal/liquid bi-stability are fabricated. In particular, the chloride complex provides a thin film exhibiting thermo- and mechano-triggered luminescence ON/OFF switching at ambient temperature, which is based on the control of the supercooled liquid phase and bright luminescent crystalline phase. The photophysical properties of the complexes are investigated and compared with those of the bromide complex and tetra(n-butyl)ammonium salts of the complexes. Furthermore, detailed photophysical analysis reveals a large contribution of the charge-transfer character to the ligand-centered (3)pi pi* excited state, which results in intense phosphorescence in the crystal and high-contrast luminescence ON/OFF by the phase transition of the chloride complex.

Interfacial electron flow is crucial for an efficient two-step (Z-scheme) solar water-splitting reaction. Dye-sensitization of a wide-gap oxide semiconductor has attracted considerable attention for decades as a means of producing hydrogen from water; however, it suffers from back electron transfer reactions at solid-solid and solid-solution interfaces. Here, we demonstrate that a combination of two nano-architectures, Ru-dye double layering and Pt cocatalyst intercalation to layered niobate semiconductor, effectively suppresses the back electron transfers at interfaces, leading to the complete oxidation of the [Co(bpy)(3)]-type electron mediator (bpy = 2,2'-bipyridine) as a result of efficient photocatalytic hydrogen production. Our systematic study on the Ru-dye double layers revealed that the double layering of two different Ru dyes and surface modification with the Zr4+ cation not only suppress the back electron transfer from an electron-injected semiconductor to oxidized dye but also accelerate the electron injection from the mediator to the oxidized dye. In addition, the re-reduction of the oxidized Co(III) mediator at the Pt cocatalyst surface was effectively suppressed by intercalation of Pt to the layered niobate semiconductor. The present work clearly shows that double nano-architectures controlling the surfaces of the semiconductor and cocatalyst have great potential for photo-induced charge separation at the solid-solution interface and expands the possibilities of layered semiconductor materials toward Z-scheme water splitting.